Closed loop fuel control systems are generally used in petroleum-powered vehicles to maintain an operating air-fuel (A/F) ratio at stoichiometry. Stoichiometric values, however, can vary with fuel composition. For example, when fuel is added to a vehicle fuel tank, it mixes with fuel already in the tank. For example, ethanol or gasohol in varying mixtures can be added to gasoline already in the tank. If the added fuel has a different composition from that of the fuel already in the tank, the engine of the vehicle may need to operate at a different stoichiometric value after the refueling. Generally, manufactured vehicles may include an emissions sensor that senses the vehicle fuel type and/or composition, and such information to other systems in the vehicle. Because an unknown fuel type and/or composition has entered and been consumed by the vehicle engine, the air-fuel ratio may be adjusted in an after-the-fact manner, which can generate unwanted emission quantities and poor performance until the air-fuel ratio can be optimized for the fuel type.